Coalescing neutron star binaries and the first stars

There is growing evidence from the observation of metal-poor stars that the rapid neutron capture elements show a unique pattern beyond Barium which excellently fits the (scaled) solar abundance pattern. However, the actual production site of r-process nuclei is still subject of lively debates. Recent nucleosynthesis calculations for the ejecta of neutron star mergers, one of the suggested production sites, have shown that the observed abundance pattern can be reproduced very accurately, if the electron fraction of the ejected material is close to 0.1. Constraints on this scenario from observations are discussed.